Highlights

Focus on Stem Cells

On the Frontline of Stem Cell Innovation

As Director of the Stem Cell Program, Dr. Leonard Zon, leads a group of more than 40 faculty and affiliate members exploring stem cell biology as a key to unlocking treatments for diseases affecting children.

Key discoveries from the Stem Cell Program include the generation of disease-specific induced pluripotent stem cells, clinical development of pharmaceuticals that regulate stem cell proliferation, techniques for the isolation and use of stem cells, and the discovery of core regulatory pathways in cancer stem cells.

George Daley, M.D., Ph.D.

Induced pluripotent stem cells

Within the swiftly moving field of stem cell biology, George Daley's laboratory is pioneering the exploration of the unique biology of pluripotent cells, particularly mouse and human embryonic stem cells and germ cells. Dr. Daley is a Howard Hughes Medical Institute investigator and was recently named associate chief and director of Pediatric Stem Cell Transplantation and the Samuel E. Lux IV Chair in Hematology. His lab is helping to define the genes and factors important for embryonic stem cell self-renewal, developing and refining directed differentiation technologies, and developing strategies for reprogramming cells to a pluripotent state including somatic cell nuclear transfer, retroviral and lentiviral targeting, homologous recombination and RNA interference.

Dr. Daley has always kept his focus on the bigger goal of reprogramming adult tissue cells from an individual patient so they revert to an embryonic state, creating cells called induced pluripotent stem cells (iPS). Once in this more primitive, pluripotent state, the adult-derived cells can theoretically be coaxed into developing as healthy replacement tissues. Dr. Daley and others are refining and testing several genetic and chemical strategies to make reprogramming more efficient.

The Daley lab has also created iPS lines from cells derived from patients with various genetic disorders. These lines will provide a valuable resource to create specific differentiated cells to study disease processes. For instance, researchers have an unlimited source of neurons from Alzheimer patients or muscle cells from muscular dystrophy patients, and they can screen for therapies that act on the specific affected cells.

Another approach to reprogram adult cells into the stem cell state relies on a technique called nuclear transfer (NT), in which the nucleus of an adult cell is placed into a egg cell with its original nucleus removed, but which still contains natural embryonic factors that reprogram the nucleus, creating a new embryo. Embryonic stem cells created from the NT-derived embryo are then grown in culture, where they can be manipulated to correct genetic mutations or induced to form specific differentiated cells. In a seminal proof of concept, the Daley group has created NT-derived embryonic stem cells from an adult mouse with a genetic blood disorder, repaired the mutation in culture, induced the cells to partially differentiate into hematopoietic stem cells (HSCs) and transplanted the HSCs to regenerate a healthy blood system in a recipient mouse. The process is extremely inefficient, but it shows potential for human therapies one day.

Another powerful technique developed in the Daley Lab to create pluripotent cells is based on parthenogenesis. This technique allows scientists to derive embryonic stem cells from an unfertilized egg — avoiding the need to use viable embryos. Eggs, or unfertilized oocytes, are artificially stimulated to duplicate their chromosomes, resulting in an embryo for cell regeneration therapy that can be tailored to be immunologically compatible with any recipient. These cells can be induced to create heart, skin, blood, liver, spleen, brain, thymus, and hair. Dr. Daley ultimately envisions “banks” of master cells and tissues for matching to specific patients, but the transplantation work is currently in a basic modeling stage in animals.

Carla Kim, Ph.D.

Lung Cancer Stem Cells

One of the few scientists in the world who's carved out a niche in lung stem cell research, Dr. Carla Kim is studying normal and cancerous lung cells. Before joining Boston Children's Hospital Stem Cell Program, she was the first to isolate bronchioalveolar stem cells (BASCs), a type of lung stem cell, from adult mice. Also, Dr. Kim found that the most common genetic mutation in lung cancer, an oncogenic K-ras mutation, appears to transform BASCs into adenocarcinoma, a typical and aggressive form of lung cancer.

Dr. Kim believes that lung stem cells such as BASCs are involved in the early, undetected stages of adenocarcinoma. This silent period can go on for years, even decades. By transplanting lung tumor cells from one mouse to another, Dr. Kim has observed that in certain cases, only tumors with BASC-like cells can grow in recipient animals. This work indicates that only a small fraction of the cells within a tumor are required for tumor growth as shown in transplantation assays. These cells may be the stem cells linked to lung cancer. Today, Dr. Kim is examining BASCs gene expression profile of the cancer stem cells to understand how they are different from normal lung stem cells. “What molecules control these cells, versus the remaining tumor, versus normal lung?” she asks. “If we can answer this, we'll have a better plan to attack lung cancer through new therapies, or combinations of existing ones.”

Beyond BASCs

Carla Kim's broader interest is to further characterize the biology of stem cells in normal tissue and in disease. She utilizes a combination of mouse genetics, cell biology and genomic approaches to explain the biology of these cells during homeostasis and tumorigenesis. In her search for other cells besides BASCs that have “stem” properties, she genetically tags likely candidates, then injures the lung to see which cell types are involved in repairing the injury, and whether these carry the same genetic tags. She hopes to learn whether normal lung stem cells could be used as a therapy for cystic fibrosis, the defective lungs of premature infants and other pulmonary problems. “There are many broad applications in lung stem cell studies that go beyond cancer,” she says.

Leonard Zon, M.D.

Clinical Development of Prostaglandins to Stimulate Hematopoiesis

The laboratory Dr. Leonard Zon, the director of the Boston Children's Hospital Stem Cell Program and a Howard Hughes Medical Institute investigator, has identified the gene cdx4, which activates homeobox genes and establishes the mesoderm's competency to make blood. His laboratory focuses on the developmental biology of hematopoiesis and uses the zebrafish as a model system to study hematopoiesis and hematopoietic stem cells, including large-scale genetic screening and chemical library screening. Dr. Zon has found that the overexpression of cdx4 leads to ectopic blood formation in the middle of the embryo. Cdx4 can increase multipotential hematopoietic progenitors in mouse embryonic stem cells. The cdx4-hox pathway thus regulates hematopoietic stem cell production by the embryo.

In zebrafish models, Dr. Zon's group has used chemical genetic screens to find compounds that modify the production of hematopoietic stem cells. From such screens, they discovered that prostaglandin E2 can increase blood stem cells during embryogenesis in adult zebrafish and in mice. A treatment of mouse bone marrow with prostaglandin leads to a threefold increase in stem cell number. Currently, Dr. Zon is leading a group that is investigating clinical trials to treat umbilical cord blood samples with a prostaglandin E2 derivative, in an effort to increase the number of stem cells available for transplants.

Dr. Zon is one of the scientific founders of Fate Therapeutics, Inc., a startup company that is developing a new approach to stem cell medicine.

Richard Gregory, PhD

Regulation of embryonic and cancer stem cells by microRNAs points to a possible therapeutic target.

A recent discovery from Boston Children's Hospital may provide a missing link between stem cell generation and carcinogenesis, and points to new approaches to blocking certain cancers. Richard Gregory, PhD, a principal investigator in the Stem Cell Program at Children's, collaborating with graduate student, Srinivas Viswanathan, from the lab of George Daley, MD, PhD, showed that the protein Lin-28 regulates an important family of microRNAs called Let-7. Central to the biology of the system is Lin-28, which is abundant in embryonic stem cells and inhibits them from differentiating into specific cell types, but which is not normally found in healthy adult tissues. Lin 28 has been used in a cocktail of genes to reprogram fully differentiated cells into induced pluripotent stem cells.

As the team reported in Science last April, increasing the level of Lin-28 protein in a cell blocked the production of mature Let-7 microRNAs, making them unavailable to regulate cell activity. Conversely, inhibiting Lin-28 production led to an increase in mature Let-7. Why is this important? Low levels of Let-7 microRNAs make a cell more prone to de-differentiate and are associated with breast and lung cancer. Let-7 microRNAs have been shown by other groups to control the expression of certain oncogenes. “We are actively seeking both drugs that mimic the effect of Lin-28 on microRNAs to enhance stem cell generation, and drugs that block Lin-28 to inhibit cancers,” says Gregory.

Today Dr. Gregory is conducting large-scale chemical screening to find such compounds. He also hopes to find other microRNAs involved in stem cell self-renewal and differentiation as well as compounds that target them.